1. Field of the Invention
The present invention relates to a resistor device for controlling rotational speed of a blower motor used for an air conditioning system, for instance, installed in an automobile.
2. Description of the Related Art
The resistor device mentioned above is disposed around an outlet of an blower of an air conditioning system of an automobile so that the resistor device is cooled by the wind from the blower.
The resistor device of the kind is disclosed in Japanese Patent Publications (KOKOKU) No.57-45041 and No.57-32482 in which a semiconductor element of positive temperature coefficient (referred to as PTC element hereinafter) is used as the resistor element of the device. The PTC element has such a characteristic, as widely known, that the resistance thereof rises abruptly and greatly when the am temperature exceeds a predetermined value (Curie temperature). As a result, when the PTC element is not appropriately cooled when the current is applied thereto or when an excessive current over an allowable extent is applied to the motor, the temperature of the PTC element is raised gradually first and when the temperature reaches Curie temperature, the resistance of the element increases suddenly very large so that the current is controlled and the temperature is maintained below the Curie temperature. Accordingly, the PTC element is very useful for proper operation of the motor and avoiding fire of the automobile.
However, according to the above mentioned Japanese patent publications (57-45041 and 57-32482) aiming at the safety structure for the motor and vehicle, the resistor device is arranged in such a manner that a plurality of ring-shaped PTC element plates and a plurality of terminal plates having a center hole are disposed alternately and combined together by bolt inserted through the center holes of the plates and secured together by a nut screwed on the bolt. In this structure, it becomes necessary to dispose an insulation spacer between the bolt and the terminal plates to avoid contact and the short circuit between the bolt and the terminal plates. As a result, the structure becomes complicated and proper adjustment of the torque for fastening the nut is necessitated, which makes the assembling work troublesome and the cost of the device becomes high.